An ignition plug with an antenna for emitting EM radiation is known. Patent document 1 describes such an ignition plug.
Patent document 1 (see FIG. 2) describes an ignition plug with an antenna located on the surface of the lower tip of an insulator. The antenna is made of an arc-like metallic foil with a predetermined width, and surrounds a center electrode, leaving space between it and the center electrode. A microwave signal is supplied to the antenna of the ignition plug from a high-pressure alternating current (AC) generator when a high voltage is applied from the ignition coil to the center electrode. In an engine employing the ignition plug, the air-fuel mixture is ignited when plasma generated by the microwave reacts with the spark discharge.    [Patent document 1] JP 2010-101174 A1
The ignition performance of a conventional ignition plug in an air-fuel mixture can be improved using a high-frequency EM wave emitted from an antenna by increasing the strength of the electric field in the electrical discharge area. This allows an internal combustion engine using such an ignition plug to reduce the pumping losses by achieving lean combustion of the air-fuel mixture and thereby improving the fuel efficiency.
The energy of the high-frequency EM wave is concentrated at the electrical discharge area and does not influence the propagation velocity of the flame. In an internal combustion engine, the amount of unburned fuel/air mixture may increase due to a decrease in the propagation velocity of the flame as the air-fuel mixture becomes lean. In an internal combustion engine using a conventional ignition plug, although the fuel efficiency can increase due to a decrease in the pumping losses, the overall fuel efficiency does not tend to increase, mainly because of the increased quantity of unburned fuel.
The present inventions are in view of this. The objective is to increase a propagation velocity of a flame by using a high frequency wave emitted from an antenna, in an ignition plug having the antenna for emitting high frequency wave to a combustion chamber of the internal combustion engine.